Lets say you've fixed a bug in a piece of software, and you now want to share your fix with the "upstream" author or distribution packager, so that you don't need to keep applying this patch to new versions, and that other people can benefit from it.
"cvs diff -u" style patch (figuring out -p properly etc). CVS also tells you what versions things are applied against, if people need to work back towards what versions you have. Otherwise (if it didn't come from cvs) do a "diff -u" from the top level directory of the /modified/ rather than in the cwd of the /original/. This means that normally you can just apply the patch with patch -p0 and it "just works".
One page links to PatchNotes:
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